Update dependency cssnano-preset-default to v6 (#308)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [cssnano-preset-default](https://togithub.com/cssnano/cssnano) | [`5.2.14` -> `6.0.1`](https://renovatebot.com/diffs/npm/cssnano-preset-default/5.2.14/6.0.1) | [![age](https://developer.mend.io/api/mc/badges/age/npm/cssnano-preset-default/6.0.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://developer.mend.io/api/mc/badges/adoption/npm/cssnano-preset-default/6.0.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://developer.mend.io/api/mc/badges/compatibility/npm/cssnano-preset-default/5.2.14/6.0.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/cssnano-preset-default/5.2.14/6.0.1?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>cssnano/cssnano (cssnano-preset-default)</summary>

### [`v6.0.1`](https://togithub.com/cssnano/cssnano/releases/tag/cssnano%406.0.1): v6.0.1

[Compare Source](https://togithub.com/cssnano/cssnano/compare/cssnano-preset-default@6.0.0...cssnano-preset-default@6.0.1)

#### Bug Fixes

-   fix(postcss-merge-rules): do not merge nested rules (cssnano/cssnano@eb9a9a1)
-   fix(postcss-reduce-idents): minify grid line names correctly (cssnano/cssnano@2af6687)

### [`v6.0.0`](https://togithub.com/cssnano/cssnano/releases/tag/cssnano%406.0.0): v6.0.0

[Compare Source](https://togithub.com/cssnano/cssnano/compare/cssnano-preset-default@5.2.14...cssnano-preset-default@6.0.0)

#### Major Changes

-   [`99d1e6a`](https://togithub.com/cssnano/cssnano/commit/99d1e6ab): postcss-normalize-url: remove normalize-url configuration options
-   [`4e272f8`](https://togithub.com/cssnano/cssnano/commit/4e272f88):  postcss-svgo: Upgrade dependency svgo to v3 and increase the minimum supported node version to v14
-   [`ca9d3f5`](https://togithub.com/cssnano/cssnano/commit/ca9d3f55): Switch minimum supported Node version to 14 for all packages
-   [`39a2040`](https://togithub.com/cssnano/cssnano/commit/39a20405): feat!(cssnano): remove undocumented YAML config support

#### Migration instructions

-   If you're not happy with the defaults for the normalize-url transform, turn it off completely. Options were removed as most would change the meaning of the URL, which is unexpcted in the case of CSS minification.
-   In the unlikely event you're using YAML to configure cssnano, move the cssnano configuration inside the PostCSS config or use a configuration file in CommonJS or JSON format.

</details>

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### Configuration

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🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

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packages/next/package.json

@@ -220,7 +220,7 @@
     "cross-spawn": "7.0.3",
     "crypto-browserify": "3.12.0",
     "css.escape": "1.5.1",
-    "cssnano-preset-default": "5.2.14",
+    "cssnano-preset-default": "6.0.1",
     "data-uri-to-buffer": "3.0.1",
     "debug": "4.3.4",
     "devalue": "2.0.1",